Embedded HTML and JavaScript

I need to stick in a banner ad in a custom page. I can enter something like this <b>aaaa</b> and it works, but if I put in a fairly complex JavaScript + HTML it fails. Now I know the script works in another web site, I’m just wodering if there are limitations of embedded it in the general text of a custom page.

[I’m using the John Hesch version of Haran]